1. Field of the Invention
The invention relates to silicone elastomers which crosslink by special rhodium compounds, a process for preparation thereof and also the use of the crosslinkable compositions.
2. Description of the Related Art
Silicone elastomers are customarily produced by crosslinking with platinum or platinum compounds.
Disadvantages of silicones which are crosslinked with platinum or platinum compounds are the yellow and/or brown color of the crosslinked silicones which are visible, in particular in the case of high silicone contents by volume. The discoloration is caused by the platinum which, at the end of the crosslinking, is present as a platinum colloid as described in the literature, J. Am. Chem. Soc. 108 (1986) 7228ff. In addition, in the case of many platinum-crosslinked silicones, the transparency is decreased and the silicone elastomers are as a result not transparent, but cloudy, which is termed translucent. The platinum catalysts have the further disadvantage that silicone rubbers comprising them have only a restricted processing time after the essential constituents are mixed, because after the individual components are mixed the platinum crosslinking begins straight away at room temperature. Although the processing time of the compositions may be increased by incorporating a substance inhibiting the activity of the platinum catalyst (these are generally termed inhibitors), the curing rate of the composition is in turn decreased as a result.
In a few cases, rhodium catalysts are also described for crosslinking silicone elastomers. U.S. Pat. No. 4,262,107 describes silicone rubber compositions which comprise silanol-endstopped polydiorganosiloxanes, silicone hydrides and rhodium catalysts. However in the case of this system, a hydrosilylation reaction does not take place, but, instead, a condensation reaction of ≡Si—OH and ≡Si—H to form ≡—Si—O—Si≡, with elimination of hydrogen. This system may be suitable for producing coatings, but it is not suitable for producing molded parts because of the hydrogen formation. DE 24 29 772 describes silicone compositions which consist of a vinyl group-containing polyorganosiloxane, a polyorganosiloxane containing silicon-bonded hydrogen atoms, and a rhodium catalyst. Rhodium catalysts used are complexes of the formula RhX3(SR2)3 or Rh2(CO)4X2, where X is halogen and R in each case is alkyl, aryl, aralkyl or alkylaryl. The resultant silicone compositions have, as advantages, long processing times after all essential constituents have been mixed together at room temperature, even without inhibitors, and good crosslinking characteristics at elevated temperatures. A great disadvantage when RhX3(SR2)3, which is described as preferred in DE 24 29 772, is used is the thioethers (SR2) which are used as ligands. These thioethers are not incorporated by crosslinking, are highly malodorous and are extremely toxic. Furthermore, these thioethers, in some cases, due to the sulfur group, lead to the resultant silicone elastomers having a yellow color. Rh2(CO)4X2 also has serious disadvantages. The most serious is certainly the fact that the compound is not stable per se in moist air and gradually decomposes. Inorg. Synth. 8 (1966), 211 ff. The rhodium complex in the silicone composition therefore is already beginning to decompose after the latter has been prepared. If it is considered that it can take several months until the silicone composition is processed at the client's premises, it becomes clear that the rhodium complex at this time is already decomposed and is no longer present in its original form, which leads to the fact that the silicone composition either no longer crosslinks at all or has completely unexpected and unwanted crosslinking characteristics.